So-called curtain airbags are known which are arranged in the roof lining of a vehicle and extend in the case of a crash after their inflation along the sides of the vehicle, so that the persons present in the vehicle are protected from injuries which can arise from the impact of a body part, in particular the head, against the side structure or the side windows of the vehicle. The lower corners of this type of curtain airbag, in particular the corner arranged at the front in the vehicle, are connected to an anchoring member via a strap or cord, usually referred to as a tether, whereby the airbag, the tether and the anchoring member form a unit that functions as a tether anchoring device. The anchoring device locks into a recess in a structural member of the vehicle. By means of a suitable mechanism, it can be ensured that the curtain airbag is sufficiently tensed and for instance an arm or head of a person can be prevented from getting caught between the airbag and the side structure or the side window of the vehicle.